Having a broadband connection on high speed rails is something that business travelers want most. Increasing number of passengers is requesting even higher access speeds. We are proposing the Media Convergence System as an ideal communication system for future high speed mobile entities. The Media Convergence System recognizes plural wireless communication media between the ground network and each train, and then traffic is load-balanced over active media which varies according to circumstances. The Media Convergence System must have a pivot wireless communication media. We are focusing on IEEE 802.11g as a pivot medium of the Media Convergence System, because it is expected to have a high performance in communication with high financial efficiency. In order to realize a high speed mobile communication system based on IEEE 802.11g (referred to as IEEE 802.11g Communication System), this paper designs the IEEE 802.11g Communication System, constructs an experimental IEEE 802.11g Communication System on a commercial high speed rail system, and evaluates performances of the system through trials.

The organization of this paper is as follows. Chapter 1 describes a background of this research and overviews this paper. Chapter 2 discusses ideal high speed communication systems in the future based on surveys of train communication technologies so far. The consideration leads a result where a Media Convergence System will be the desired high speed mobile communication system for the future. Chapter 3 through Chapter 7 is divided into two parts. Part 1 consists of Chapter 3 and Chapter 4, in which they stress a necessity of a pivot wireless communication medium of the Media Convergence System and explains that IEEE 802.11g satisfies all the requirements to be the medium. The IEEE 802.11g Communication System is proposed, designed and constructed on a commercial high speed rail system, and then performances of that system are evaluated in this part. Trial results have proven that the IEEE 802.11g Communication System realizes the maximum application throughput of around 20(Mbps). On the other side, the results have also clarified that the IEEE 802.11g Communication System suffers from very frequent Layer 2 Handovers (L2HO) which degrade the communication quality. Part 2 consists of Chapter 5 through Chapter 7. In order to solve problems caused by L2HOs, Part 2 proposes the Bicasting-Multipath Mobile IPv4 and it is verified through trials. Trial results have proven that the Bicasting-Multipath Mobile IPv4 improves the communication quality over L2HOs and also stabilizes communication.

Detailed discussions in each chapter are as follows. Chapter 1 describes a background of this research and overviews this paper. The Internet using TCP/IP is not only for immobile computers in rooms but also for any devices under any situation for anyone who wants to use it. It is not an exception when people are on a high speed train traveling at around 300(km/h). Although we have demands for connectivity to the Internet from cabins, a supply of communication bandwidth is not enough for the demands in high speed mobile environments. Chapter 1 explains a necessity of making communication bandwidth broader for high speed mobile systems.

Chapter 2 surveys various train communication systems with their history and discusses present systems. The focus of this paper is communication on high speed rail systems. High speed trains generally travel through various geographical areas such as cities, suburbs, mountains, tunnels and so on. With these geographical conditions taken into account, there seems to be no perfect wireless communication medium which suits best to all places. Communication media which have different characteristics from each other should be deployed in accordance with geographical conditions. Plural communication media may become available in the same place at the same time. A wireless communication system for high speed trains in the future must recognize these plural wireless communication media and traffic must be load-balanced over active media which varies according to circumstances. It is referred to as "Media Convergence System" that all the communication media available at local places are logically bundled up into a single path. This chapter proposes a Media Convergence System which will be desired in future high speed trains.

Chapter 3 discusses a pivot communication media of the Media Convergence System. The pivot wireless communication medium for the Media Convergence System has to satisfy four following requirements.

(1)To have enough communication bandwidth

(2)To be feasible and affordable in the near future

(3)To be easy to procure devices for the system

(4)To be easy to operate

After a careful consideration of these requirements, IEEE 802.11g was employed as a pivot wireless communication medium in the Media Convergence System. This research is aiming at developing a broadband communication system based on IEEE 802.11g under high speed mobile environments.

Chapter 3 also designs the IEEE 802.11g Communication System for each layer. Major design policies for each layer are as follows. For a radio transmission channel on Layer 1, a radio propagation line was established on the train track by a high gain directional antenna. The Fresnel Zone was kept as clear as possible to make the quality of the radio transmission channel higher. As for Layer 2, the IEEE 802.11g Communication System leverages IEEE 802.11g as the name indicates. One of the requirements for the system is to construct a system in which it is affordable with the use of IEEE 802.11g. Our policy does not permit any customizations on IEEE 802.11g. Product dependences were accepted on Layer 2. As for Layer 3, design policies are as follows. The IEEE 802.11g Communication System assumes trains as its mobile stations. Since trains move on their tracks, a migration path for each train can be identified unless the train suddenly leaves its intended route. This peculiar characteristic allowed us to design a network topology which minimizes damages to communications caused by L3HOs.

Chapter 4 verifies that the IEEE 802.11g Communication System performs in accordance with the designing intentions and proves that the system works in real situations. The trials were done on a commercial high speed rail system. The speed of the test train was kept at 270(km/h). Experimental results have revealed communication performances on the IEEE 802.11g Communication System as shown in Table. 1. Results showed that the IEEE 802.11g Communication System working in Mode 2 had a TCP bandwidth of 13.7(Mbps) even while a mobile node was moving at 270(km/h). On the other side, however, Table. 1 also clarified a problem of high communication stall duration rate caused by downtimes over L2HOs and wireless link failures. Here, a ratio of communication stall duration to the whole communication period is referred to as Communication Stall Duration Rate.

In order to reduce Communication Stall Duration Rate, requirements to improve the IEEE 802.11g Communication System were considered. Our policy does not permit any customizations on the IEEE 802.11g. Instabilities on Layer 1 and Layer 2 must be compensated for by Layer 3. These discussions brought three Requirements to be satisfied as follows.

(1)To enable traffic forwarding while a Layer 2 link is down

(2)To eradicate TCP Time Out caused by L2HOs

(3)To be applicable to all transport protocols

In order to improve the IEEE 802.11g Communication System with satisfying these three Requirements, Chapter 5 proposes "Multiplexing of wireless links between the ground network and each train network" and "Bicasting traffic between the redundant paths." Two wireless links between the ground and a train were redundantly established. Each link was recognized as different IP routes (Paths) by MP-MIPv4. This is a bicasting architecture of traffic over two MIPv4 tunnels. The architecture was named "Bicasting-Multipath Mobile IPv4."

Chapter 5 describes our proposal of the Bicasting-Multipath Mobile IPv4. A MP-MIPv4 network for its platform was also designed and the characteristics were considered with experimental results. The results have shown that IPLB (IP-based Load-Balancing) is the best load-balancing algorithm for the IEEE 802.11g Communication System.

Chapter 6 describes a design and an implementation of a bicasting architecture between paths in the Bicasting-Multipath Mobile IPv4. The bicasting architecture works over redundant paths established by MP-MIPv4. In order to satisfy three Requirements of the IEEE 802.11g Communication System held up in Chapter 4, the Bicasting-Multipath Mobile IPv4 bicasts traffic while either a L2HO or a Wireless Link Failure is taking place. Here, bicasting has to be started before the occurrence of either of them. Even if bicasting is started after a detection of a link being down, target traffic is no longer there and thus TCP has to wait for a RTO expiration. Therefore the Bicasting-Multipath Mobile IPv4 has to predict both a L2HO and a Wireless Link Failure before the start of its process.

The IEEE 802.11g Communication System predicted the events by RSSI (Received Signal Strength Indication) which each TBR received from one of the GBRs. RSSI on both TBRs were observed every 125(ms) by SNMP get-requests. For the Front BR which was set up at the front edge of the test train, a L2HO was predicted by an occurrence of a RSSI peak. This method successfully predicted L2HOs on the Front BR with the rate of 88.2%. For the Rear BR which was set up at the rear edge of the test train, on the other side, we introduced a threshold to judge a L2HO. The threshold is referred to as "L2HO threshold." When a RSSI fell under this L2HO threshold, a L2HO was predicted. Accuracy on the Rear BR depends on a configurable L2HO threshold.

Chapter 7 reports experimental results of the Bicasting-Multipath Mobile IPv4. Communication performances on the Bicasting-Multipath Mobile IPv4 are shown in Table. 2. The results have proven that the Bicasting-Multipath Mobile IPv4 is able to reduce Communication Stall Duration Rate to 0.67% and realizes TCP Rate of 16.4(Mbps) at the same time. Our proposed method improved the Communication Stall Duration Rate which was a serious problem of the Singlepath Configuration. The Bicasting-Multipath Mobile IPv4 realized both high communication speed and low Communication Stall Duration Rate at the same time. We have successfully configured a practical communication system for high speed rail systems.

Table. 1Communication Performance on the IEEE Communication System

Table. 2Communication Performance on Bicasting-Multipath Mobile IP

本論文は「IPレイヤにおけるバイキャスティングアーキテクチャを実装した高速移動通信システム」(A High Speed Mobile Communication System implementing Bicasting Architecture on the IP Layer)と題し,現在の高速移動通信システムの通信帯域が求められる要求に対して十分ではないとの認識に基づき,実現すべき高速移動通信システムの将来像を示した上で,それを実現するための具体的手法として,IEEE 802.11gを利用した通信システムを提案している.IEEE 802.11gは本来,固定または低速の移動体向け通信用に開発された無線通信メディアであるが,通信性能だけでなく経済性を考慮に入れる鉄道事業者の視点で,経済性に優れるIEEE 802.11gを高速移動に適用するため,ハンドオーバ時にIPレイヤでバイキャスティングする手法を提案・構築し,実証実験の結果を示して,提案システムの実用性を明らかにしている.本論文は,英文で記述され,8章から構成されている.

第1章は「Introduction」で,高速移動環境からのインターネットアクセスニーズが益々高まっている一方,地上と高速移動体を接続する無線伝送路の通信帯域が地上ネットワークに比較して大きく劣っており,その広帯域化が求められているという本研究の背景と概要を述べている.

第2章は「Technological Surveys of Train Communication Systems and the Ideal Communication System for the Future」と題し,鉄道通信システムの現状を俯瞰し,唯一の無線通信メディアにより高速鉄道通信を実現することの困難を指摘し,高速鉄道における将来目指すべき無線通信システムについて議論している.高速鉄道は,都市,郊外,山間部,トンネルなどさまざまな地形を移動するという地理的条件から,最適な無線通信メディアは移動する全域にわたって唯一ではないとの考察を行っている.その結果,高速鉄道における将来目指すべき無線通信システムでは,地上局~移動局間に複数の無線通信メディアを構築し,列車の移動とともに変化するアクティブな無線通信メディアの間でトラフィックの分散送信を行うMedia Convergence Systemを提案している.

第3章は「Proposal of a High Speed Communication System based on IEEE 802.11g」と題し,Media Convergence Systemにおいては通信性能が不十分な通信メディアを結合しても,満足できる通信パフォーマンスを得ることはできないとし,Media Convergence Systemにおいて軸となる通信メディアの必要性を述べている.この通信メディアとして,通信性能,経済性に優れるIEEE 802.11gを選定し,IEEE 802.11gを利用した高速移動通信システムの設計を行っている.設計では,レイヤ2にIEEE 802.11gを利用するため,IEEE 802.11gの規格自体には改良を施さず,レイヤ1とレイヤ3を適切に設計することで,目標とする10MbpsのTCPスループットを実現する手法を提案している.レイヤ1の設計においては,高利得アンテナを開発するとともに,通信点間に構成されるフレネルゾーンを丁寧に解析した上で,高速移動環境における実測データを自由空間モデルと比較・評価している.試験結果より,高速移動環境でも10MbpsのTCPスループットを実現するための目安となる-81(dBm)の電界強度を得るためには,地上局間隔を500(m)程度とするのが最適であることを導出している.レイヤ3の設計においては,移動局となる鉄道車両の移動経路が事前予測可能であるという特性を最大限利用し,データトラヒック転送中にMobile IPのレイヤ3ハンドオーバを実行する手法を提案している.

第4章は「Communication Performance Evaluation of the IEEE 802.11g Communication System based on a Singlepath Configuration」と題し,第3章で設計したIEEE 802.11gを利用した高速移動通信システムを東海道新幹線に構築し,実証実験を実施した結果を報告している.実証実験の結果,提案システムの通信パフォーマンスは,平均RTTが9.95(ms),平均TCPスループットはMode 1で9.86(Mbps),Mode 2で13.7(Mbps)であった.なお,Mode 1とは移動局が通信中の地上局に接近しながら通信するモード,Mode 2とは移動局が通信中の地上局から離脱しながら通信するモードである.また,高速移動に起因してIEEE 802.11gリンクが5.2%の確率で無線リンクの構成に失敗することを報告している.この無線リンク障害とレイヤ2ハンドオーバにおけるTCPの動作を詳細に解析し,無線リンクの切断だけが課題ではなく,TCPがRTOを待つことによる自発的な通信中断によりTCPが通信不能に陥ることを指摘している.この通信不能時間率について統計的に算出しており,Mode 1で9.9%,Mode 2で18.1%であった.この時間率の低減について課題提起し,通信システム改善のための要求条件として次の3項目を挙げている.(1)レイヤ2のリンクダウン中においてもトラヒックフォワーディングが可能であること,(2)レイヤ2ハンドオーバに起因するTCPのタイムアウトを発生させないこと,(3)すべてのトランスポート層プロトコルに適用可能であること.

第5章は「Proposal of the Bicasting-Multipath Mobile IPv4」と題し,第4章で課題提起したTCPの通信不能時間率低減のための3つの要求条件を満足させるためBicasting-Multipath Mobile IPv4を提案している.これは地上・車上間に2本の無線リンクを冗長的に構成し,Multipath Mobile IPv4によりそれらを別のIPルート(パス)として識別し,2本のMIPv4トンネル相互間でバイキャストを行うアーキテクチャである.Multipath Mobile IPv4プラットフォームについて解説するとともに,2本のパス上におけるトラヒックの負荷分散方法について,(1)パケット毎の負荷分散,(2)宛先IPアドレスに基づく負荷分散,を比較検討し,(1)パケット毎の負荷分散ではパケット到着順序の逆転をTCPがパケットロスと判断し不用意にウインドウ制御を実行する問題を指摘し,(2)宛先IPアドレスに基づく負荷分散が望ましいと結論づけている.

第6章は「Bicasting Architecture of the Bicasting-Multipath Mobile IPv4」と題し,Bicasting-Multipath Mobile IPv4におけるバイキャスティングアーキテクチャの設計を行うとともに,通信システムへのBicasting-Multipath Mobile IPv4の実装手法を詳説している.特にレイヤ2ハンドオーバにおけるバイキャストの制御については検討している.レイヤ2ハンドオーバが開始された直後にTCPは送信を停止するため,レイヤ2ハンドオーバを検知した後にバイキャストを開始しても対象トラヒックはすでに存在せず,TCPはタイムアウトを待たなければならないことを指摘し,レイヤ2ハンドオーバの予測に基づいてバイキャストを開始する必要性を述べている.車上局が受信する電界強度を繰り返し測定し,レイヤ2ハンドオーバの直前には特徴的な変化があることを発見し,その動作をトリガとしてレイヤ2ハンドオーバを予測する手法を提案している.提案手法では車上局がMode 2で動作する際には88.2%の精度で,Mode 1で動作する際には設定パラメータに依存するもののMode 2と同程度の精度でレイヤ2ハンドオーバの予測が可能なことを明らかとしている.

第7章は「TCP Performances in the Bicasting-Multipath Mobile IPv4」と題し,第5章,第6章で提案・設計したBicasting-Multipath Mobile IPv4を,IEEE 802.11gを利用した高速移動通信システムに実装し,東海道新幹線を利用して実証実験を行った結果を報告している.実験結果より,第4章で掲げた3つの要求条件を満足した上で,16.4(Mbps)の平均TCPスループットを実現するとともに,通信不能時間率を0.67%にまで低減したことを明らかとしている.

第8章は「Conclusion」で,本論文を総括している.

以上のように,本論文は,将来の高速鉄道通信システムの理想型としてMedia Convergence Systemを明示するとともに,Media Convergence Systemの軸となる無線通信メディアとしてIEEE 802.11gを利用した高速移動通信システムを提案している.その上で,IEEE 802.11gを利用した高速移動通信システムを実現するため,Bicasting-Multipath Mobile IPv4を提案し,その実用性を東海道新幹線という実環境下で実証実験により明らかにしている.提案手法はIPレイヤにおけるバイキャスティングにより移動通信の安定性向上に寄与するものであり,鉄道事業者において重要なだけでなく,電子情報学上貢献するところが少なくない.

よって本論文は博士(情報理工学)の学位請求論文として合格と認められる.